Radio frequency (RF) power amplifiers are used in a wide variety of communications and other electronic applications, such as cellular handsets and base radio repeaters. Power amplifiers typically have several cascaded amplifier stages, each of which is formed from one or more power transistors. An amplifier stage may contain a discrete transistor, or a power integrated circuit (IC) as the primary active device. The terms RF power amplifier device, power amplifier device, transistor, power IC, or IC are used interchangeably herein to describe a device that contains one or more power transistors.
Recent technological advances of power amplifiers have increased the operational frequency bandwidth of the power amplifiers to accommodate a broader range of applications. A single exemplary 2 GHz power amplifier or power amplifier transistor of sufficient bandwidth can be used in designs for some or all of: the GSM 450, 480, 850, and 900 MHz bands; the SMR iDEN 800 MHz band; AMPS, GSM, IS-95 (CDMA), IS-136 (D-AMPS) Cellular band; GSM, IS-136 (D-AMPS) PCS band; and 3 G AWS bands in the 700 MHz range and at 1432-1435, and 1710-1755 MHz. Other bands, such as those below the 450 MHz range may also be amplified. Companies that develop products for applications that span several frequency bands may, for reasons such as cost or volume considerations, desire to use a single power amplifier in multiple products.
Although power amplifiers have maximum rated output voltage levels, these levels can be exceeded even under nominal operating conditions if proper design precautions or protection elements are not utilized. The term output voltage as used herein refers to the voltage across the drain and source of a field-effect transistor (FET) or the collector and emitter of a bipolar junction transistor (BJT). Exceeding the maximum output voltage levels could result in undesirable consequences such as device failure, rendering the power amplifier partially or fully disabled, or compromised personal safety due to fire. The maximum rated output voltage levels can be exceeded, for example, if an output network connected to the power amplifier is not optimally impedance matched to the power amplifier, or if an RF overdrive event occurs. An RF overdrive event is defined by abnormally high RF input power to the power amplifier.
An RF overdrive event may occur when the power amplifier amplifies an input signal of a sufficiently low fundamental frequency. More specifically, when the power amplifier amplifies signals at sufficiently low fundamental frequencies below the maximum operating frequency of the power amplifier, one or more harmonics of the fundamental frequency can lie within the operational bandwidth of the power amplifier device. Harmonics are herein defined as integer multiples of the fundamental frequency. The contributions of the harmonics to the composite output voltage containing the fundamental and harmonic voltage components can be significant and can result in exceeding the maximum rated output voltage of the transistor even at nominal power levels within the rating of the device. Especially in power amplifiers to be used in applications that span several frequency bands, the gain bandwidth of the power amplifier may be limited to a narrow band of operation yet still be prone to the device failure if the device gain is of sufficient bandwidth to contain harmonics. As above, this can result in permanent damage to the device or decrease the product life.
For an exemplary VHF band of 150 to 174 MHz, a 1 GHz operational bandwidth device includes harmonic frequencies through the 6th harmonic of the fundamental. The peak output voltage can be notably affected by any or all of these frequency components. If the resulting voltage waveforms of multiple frequency components lie in phase with respect to one another, the composite peak output voltage can increase significantly over that of the fundamental frequency component alone.
In applications such as base station transmitters it is further desirable to linearize the output of the power amplifier. One common technique to maintain linearization of the power amplifier uses Cartesian feedback. Training sequences are performed periodically over the lifetime of the system to condition the feedback loop for the gain and phase of the feedback signal so that the loop maintains stable operation. An RF overdrive event can occur when the Cartesian feedback becomes unstable, which may occur due to software glitches and/or hardware performance. The instability results from a change in loop dynamics occurring between loop training sequences. These loop-instabilities can occur at various points throughout the lifetime of the system due to internal influences, such as age-based changes in the individual devices within the loop, or external influences, such as changes in the ambient temperature. In order to meet linearity requirements, gain stages in the transmitter typically have saturated power levels above that required for nominal average power operation. Because of this, a loop-instability can produce abnormal drive levels to a given amplifier stage due to the headroom of the preceding amplifier. If proper precautions are not taken when designing the power amplifier, failure of one or more of the elements contained within the power amplifier may occur when an RF overdrive event causes the instantaneous output voltage on an RF power device to exceed its breakdown voltage.